Solid state radio frequency (RF) switches are important components of Radar transmit/receive (T/R) modules, satellite communication systems, Joint Tactical Radio Systems (JTRS), and the like. A promising RF switch technology uses Heterostructure Field Effect Transistors (HFETs). Recently, high power switches made of AlGaN/GaN HFETs demonstrated superior performance over other RF switching devices in terms of maximum power density, bandwidth, operating temperature, and breakdown voltage.
Many applications, including JTRS and low-noise receivers, require RF switches with a very low insertion loss, e.g., typically below 0.3 decibels (dB). A low loss switch dissipates little RF power. As a result, it can be fabricated over a low cost substrate, such as sapphire. Low insertion loss in an HFET is due to a high channel conductance of the device, which is proportional to a total length of the device periphery. Exceptionally high 2D electron gas densities at the AlGaN/GaN interface make a group III-Nitride HFET with a total periphery of two to five mm an ideal candidate for RF switching applications.
The feasibility of high-power broad-band monolithically integrated group III-Nitride HFET RF switches has been demonstrated. Large signal analysis and experimental data for a large periphery group III-Nitride switch indicate that the switch can achieve switching powers exceeding +40 to +50 dBm.
Quality ohmic contact formation is a significant problem in the manufacture of wide band gap semiconductor devices. In particular, high temperature annealing (e.g., at 850-900 degrees Celsius) leads to material degradation during post-growth processing in the manufacturing process. For operating frequencies in the gigahertz range, group III-Nitride-based RF switches can be manufactured using capacitively-coupled contacts. The manufacture of such capacitively-coupled contacts can be performed using a self-aligned metallization process that does not require contact annealing. This process provides a reduction in material degradation and simultaneous formation of RF input, output and control electrodes. RF switches with capacitively-coupled contacts can be manufactured using either Schottky electrodes deposited over a wide band gap barrier layer or metal-oxide-semiconductor (MOS) structures with electrodes formed over oxide-semiconductor heterojunction films.
The resulting RF switch with capacitively-coupled contacts can comprise a very low contact resistance at RF frequencies, particularly frequencies of approximately two gigahertz and above. Additionally, such group III-Nitride switches have been shown to be capable of low-loss high power RF switching. For example, a group-Ill Nitride RF switch with capacitively-coupled contacts has provided insertion loss below one decibel with isolation of thirty decibels in the frequency range of one to eleven gigahertz.